1. Technical Field
The present invention relates to a screw, and more particularly, to a screw having improvements in screw head.
2. Description of the Related Art
Reference is made to FIGS. 1 and 2, which illustrate a conventional flat head screw 10 having cutting ribs. The flat head screw 10 includes a shank 11 and a flat head 12 with a conical shape. The shank 11 connects with the head 12 at one end and gets tapered at the other end. Four cutting ribs 13 are equally positioned on the conical side-surface 15 of the head 12 in longitudinal directions. Reference is now made to FIG. 3a, which illustrates that two pieces of plates 16, 18 made of wood, acrylic or plastics are fastened together by the screw 10. For illustration purpose, the plates 16 and 18 illustrated in FIG. 3a are two pieces of wood. The wood 16 stacks up on the wood 18 and the screw 10 is driven to first drill the wood 16. The screw 10 will drill through the wood 16 to wood 18. The four cutting ribs 13 will cut the wood 16 and finally the head 12 is countersunk into the wood 16 when the screw 10 continues to be drilled. This way the screw 10 fastens the woods 16 and 18 together.
However, the above conventional screw 10 has drawbacks as follows: First, the four cutting ribs 13 at the head 12 are curved and therefore have poor performance in cutting. This causes the head 12 hard to be countersunk into the wood 16 and it is required to use a larger torque to drill the woods accordingly. Secondly, since the four cutting ribs 13 are positioned on the conical side-surface 15 of the head 12 in the longitudinal directions, it causes the cutting ribs 13 poor performance in cutting. This renders the screw 10 not suitable for fastening a hard workpiece. Finally, since the four cutting ribs 13 are curved and positioned on the conical side-surface 15 of the head 12 in the longitudinal directions, it often causes the wood grain to be twisted surrounding the head 12 when the head 12 is countersunk into the wood 16 that is a hard wood having hard fibers or is a complicated compound wood. In addition, it may also cause the wood 16 to be cracked, have burrs or form a bulge 14 surrounding the countersunk head 12. Reference is now made to FIGS. 3b, 3c and 3d, which respectively illustrate the wood 16 has cracks, twisted grains and burrs surrounding the head 12 of the conventional screw 10 when the head 12 is countersunk into the wood 16.
In order to solve the above-mentioned problems and ensure the head 12 of the screw 10 to be countersunk into the wood 16 completely, it is common to suggest pre-drilling a hole in the wood 16 before fastening the screw 10 to the wood 16. The pre-drilling may be carried out by using a drilling tool with blades. The drilling tool may drill an enlarged hole by its blades on the upper surface of the wood 16. When the screw 10 is fastened to the wood 16 through the enlarged hole, the enlarged hole may receive the head 12 of the screw 10. This way the above-mentioned problems may be solved. However, it is troublesome to pre-drill an enlarged hole with a drilling tool.
Accordingly, there exists a need to provide a solution to solve the aforesaid problems.